The plastic sheet forming technique is simple and easy to realize, that is why, it is widely used for packaging commodities. Similarly, in In-Mold-Decoration (IMD) molding technology, due to the complex geometry of the membrane and the high requirement of the dimensional accuracy, geometric design and molding technique for the product should be focused on controlling the thickness distribution of shell or membrance plastic products in order to achieve high precision manufacturing. This paper started with analyzing the performance data of the plastic sheet molding material, using nonlinear finite element method and multi-physics coupling method to simulate the plastic sheet forming process, and the result gives the required parameters for product design and quality control. For the thickness deviation, the experimental testing shows that the maximum discrepancy between the simulation and actual result is less than15%. The research proved that computer simulation can contribute to control the inhomogeneity of the shell or membrane so as to improve the design and the quality of manufacturing.